T-Mobile executives explained that the operator began laying the groundwork for its buildout last year, before the company's merger with Sprint was completed, which is how it was able to turn on 5G in Sprint's 2.5GHz spectrum in Philadelphia and New York City just weeks after it closed its merger.

"I think we've all surprised ourselves at how quickly we've been able to get work done," said T-Mobile network chief Neville Ray during an appearance at an investor conference Tuesday.

Importantly, even during the COVID-19 pandemic, Ray said T-Mobile has been hanging 2.5GHz transmission radios on its existing cell towers at the rate of roughly 1,000 towers per month – a rate he said the operator expects to increase as it moves into the summer.

"So far so good," he said. "We are ramping up in this [pandemic] environment, not actually slowing down."

Prior to the close of T-Mobile's merger with Sprint, the operator counted a total of roughly 65,000 macro cell towers across the country, explained Ray. Now that its merger with Sprint is complete, he said the company's goal is to grow that number to a total of 85,000 macro cell sites by building 15,000 new cell towers and decommissioning an unspecified number of unnecessary, overlapping Sprint cell sites. Those figures roughly align with the plan the operator laid out in 2018 when it announced its merger with Sprint.

T-Mobile is hoping to install 2.5GHz radios on around 30,000 towers in major metropolitan areas quickly – though Ray did not specify a timeframe for that effort – and then later the company would work to construct the additional 15,000 new towers it will need.

T-Mobile is currently in discussions with tower companies to complete that work. "We're looking to drive some healthy competition between those folks," said Ray.

It often takes five to ten days for tower work crews to add the necessary equipment to a tower, but it frequently takes months to obtain the proper permits from local regulators, Ray explained.

The addition of Sprint's midband 2.5GHz spectrum to T-Mobile's lowband 600MHz 5G network is "really going to deliver an incredible 5G experience," Ray said, noting that the operator is already seeing speeds of 600-700Mbit/s on early operations.

T-Mobile's high-speed 5G coverage area will quickly dwarf those from its rivals, according to Ray. Interestingly, he said T-Mobile's 2.5GHz 5G network in Philadelphia, completed in a month, is roughly 2.5x larger than the 5G network running in millimeter wave (mmWave) spectrum that Verizon has constructed in roughly 30 cities over the past 18 months.

However, to be clear, a mmWave 5G network requires the installation of a much larger number of transmitters than a 2.5GHz network, because signals in 2.5GHz travel much further than signals in mmWave spectrum. The benefit of mmWave 5G is that it can support speeds well above 1Gbit/s.

In his comments, Ray didn't mention T-Mobile's small cell plans. The company today counts around 26,000 small cell/distributed antenna system sites, and T-Mobile in 2018 said it would increase that number to around 50,000 after the close of its Sprint merger.

Finally, T-Mobile CEO Mike Sievert pointed out that the operator's 5G buildout won't rely exclusively on its lowband 600MHz and midband 2.5GHz spectrum licenses. Indeed, while it was working to close its Sprint merger, T-Mobile spent roughly $1.7 billion collecting mmWave spectrum licenses across the country during the FCC's three recent mmWave spectrum auctions.

"Our competitors obviously spent a lot of time trying to convince everybody that 5G was synonymous with mmWave," Sievert said. Verizon has made mmWave 5G the centerpiece of its marketing. "We've always been fans of mmWave, as an augment."

T-Mobile's end goal – one that Sievert said the operator would spend up to $60 billion over the next five years reaching – is to deploy a "layer cake" 5G network across the US. That layer cake will include lowband spectrum for coverage, midband spectrum for capacity inside cities, and highband, mmWave spectrum for extra-fast speeds in dense urban areas. T-Mobile has already deployed this kind of network in New York City.